Most of the processes for separating aromatics from hydrocarbon mixtures such as catalytically reformed gasoline or hydrogenated pyrolysis gasoline are liquid-liquid extraction and extractive distillation, in which the most commonly used selective solvents in liquid-liquid extraction are glycerols or sulfolane derivatives. Although the liquid-liquid extraction process can obtain benzene, toluene, and xylene (BTX) product having high purity with a high yield, the prior liquid-liquid extraction processes are only suitable for treating feedstocks containing 30–70 wt % of aromatics. When treating feedstocks containing aromatics more than 70 wt %, raffinate oil has to be added; thereby, more energy is consumed and the efficiency of the device is lowered.
Extractive distillation is a process for separating aromatics from hydrocarbon mixtures which takes advantage of the solvent having the different effects on the relative volatility of various components in hydrocarbon mixture. There is no need to add raffinate oil into hydrocarbon mixtures when feedstocks containing high content of aromatics are treated. The solvent used is generally pure N-formyl morpholine, N-methyl pyrrolidone, or sulfolane, etc. For example, U.S. Pat. No. 5,310,480 (incorporated by reference in its entirety) disclosed a process for separating aromatics by extractive distillation, which uses only one column containing a reboiler and a cyclone to separate aromatics from non-aromatics in the feedstock. The upper section of said column is used for extractive distillation, and the lower section is used for recovering solvent. In order to enhance the selectivity and moderate the severity in solvent recovery, 0.1–20 wt % of water is added into the selective solvents such as sulfolane derivatives, glycerols, or glycerol ethers, while the procedure of solvent recovery is performed by means of steam stripping. Said patent teaches that the selective solvent for aromatics may be a composite solvent, wherein one component is a polyalkylene glycerol having low molecular weight, and another component is a glycerol ether, and the mutual solubility of the two components is 0.1–99 wt %. The aromatics obtained from this extractive distillation process cannot directly serve as a product for containing water, due to the water-containing solvent and the steam stripping used therein; therefore, further distillation is needed. Two liquid phases are readily formed when a feedstock which a low content of aromatics is subjected to extractive distillation due to the restriction of the solubility in the water-containing solvent, which thereby affect the normal operation. In addition, the water contained in the system tends to cause corrosion of the device.
CN1262264A disclosed a combined process of extractive distillation and liquid-liquid extraction, which pre-distillates the feedstock to yield a first fraction containing benzene and a second fraction containing small amount of benzene. The first fraction is subjected to extractive distillation to yield the benzene product, and the second fraction is mixed with the raffinate obtained from extractive distillation and then is subjected to liquid-liquid extraction to obtain toluene and a small amount of benzene. This process has less restriction on the feedstock, broader applicability, lower energy consumption, and can recover BTX simultaneously, which is therefore suitable for expending and reforming of the existing liquid-liquid extraction devices in the refinery. In said patent, 0.1–15 wt % of C8–C10 aromatics are added into the selective solvent for separating benzene by extractive distillation as a solutizer to increase the solubility of the solvent, which thereby avoids the adverse effect of water on the recovery of benzene, since both the solvent and the system contain no water. However, said invention only puts the situation where o-xylene serves as a solutizer into practice, and is limited within the extractive distillation of a combined process.
In the prior processes of using a single extractive distillation solvent, the temperature of the recovery column cannot be too high due to the restriction of the decomposition temperature of the solvent during the recovery of solvent; therefore, the level of vacuum should be raised to ensure a thorough separation of the solvent and aromatics in the rich solvent; as a result, the operation severity of the recovery column is relatively high. Besides, the extractive distillation solvent often contains O, N, or S atoms since a certain polarity is required to increase the selective solubility of aromatics; therefore, the distillation solvent has certain weak acidity or basicity, which imparts the aromatic product having corresponding weak acidity or basicity. Therefore, the product thus obtained has to be subjected to a post treatment to neutralize the acidity or basicity.